Laser-beams of different wavelengths (colors) are commonly used in a broad range of life-science applications from scientific research leading to medical advances, to clinical functions where lasers are used in diagnostics. Life science applications include flow cytometry and cell sorting, multi-spectral microscopy, DNA sequencing and retina scanning. The laser-beams are predominantly used in these applications to excite fluorophores attached to a sample, allowing researchers and clinicians to perform a variety of important tests at both the cellular and molecular level.
A common method of transporting laser-radiation to an application site, microscope or apparatus is via an optical fiber. In applications such as multi-spectral microscopy or flow cytometry, which require radiation in a range of different wavelengths, laser-beams of these different wavelengths must be coupled into the optical fiber from sources providing the different-wavelength laser-beams.
Semiconductor laser modules delivering radiation at wavelengths in a range from the ultraviolet (UV) region to the near infrared (NIR) region of the electromagnetic spectrum are commercially available. In such modules, radiation from one or more diode-lasers is coupled into an optical fiber for delivery. If a plurality of such sources is relied on to produce a plurality of different-wavelength beams for a particular application, means must be provided for combining the different-wavelength beams into single beam, usually transported by a single optical fiber as noted above.
Prior-art means for combining beams of different wavelengths have involved using a dispersive element such as a prism or diffraction grating “in reverse”, i.e., by delivering beams to the element with an angle therebetween which corresponds to the deviation angle of those wavelengths where a beam having a continuous spectrum including those different wavelengths is dispersed by the element. Where optical losses are of concern, a prism is usually preferred. This is because a prism configured to combine beams at the angle of minimum dispersion (incidence angles at entrance and exit faces of the prism about equal) can transmit plane-polarized radiation (p-polarized with respect to the prism faces) with reflection losses less than about 1% with negligible anamorphic distortion. By way of example, an isosceles prism of F2 glass with an apex angle of 60° (an equi-angle prism) has a minimum deviation angle of about 55°, which is close to the Brewster angle of 56°.
A problem with combining fiber-delivered beams is that fiber terminations such as GRIN lenses, ferrules or connectors determine a minimum spacing for adjacent delivery fibers. A prism at minimum dispersion provides a relatively low dispersion which means that the delivery fibers must be located a relatively long distance from the prism to deliver along the appropriate dispersion angle. In the case of the F2-glass prism exemplified above, visible-spectrum wavelengths from blue at 405 nanometers (nm) to red at 640 nm are included within an angle of about 3°.
A minimum convenient spacing for FC fiber-connectors on a beam combiner housing would be about 5 millimeters (mm) center to center. If it were desired to combine five wavelengths including the red and blue wavelengths, using five connectors, that would require to a spacing between the red and blue wavelength connectors of about 20 mm. This would require that the five connectors be spaced at about 40 centimeters (cm) from the prism. In order to fit such an arrangement in a convenient enclosure having a “footprint” less than say 10 cm by 10 cm, this would require “folding” the beam path between the connectors and the prism using as many as four fold mirrors. Each fold mirror would require a broad-band high-efficiency reflective coating for p-polarized radiation which would add to the cost and complexity of the combiner. There is a need for an alternate approach to accommodating a single beam-combining-prism into a fiber connected multi-wavelength beam combiner of convenient dimensions.